1. Field of the Invention
This invention relates to a semiconductor memory device, and more particularly to the improvement of data access time.
2. Description of the Related Art
In a computer, the operation speed of a memory cannot follow the improvement of the operation speed of a central processing unit (CPU) and a difference between the operation speeds thereof tends to become larger. Therefore, it has been required to develop a memory having a shorter data access time.
A page mode is provided as an operation mode for enhancing the operation speed. The page mode is a method for designating a column address with one row address kept unchanged.
Since, in the page mode, one row is set in the selected state, data can be read out simply by designating a column address so that time required for row selection can be omitted and the data access time can be reduced.
However, in the page mode, only one row can be selected and requested data access is not always associated with the selected row. Therefore, each time a different row is selected, time from selection of the row until data is output from the memory becomes longer.
In the page mode, the probability (hit rate) that data access associated with the selected row is requested (this is hereinafter referred to as "hit") is low. Therefore, the average of data access times taken when data access to a row different from the selected row is requested (this is hereinafter referred to as "miss") and access times taken in the case of "hit", that is, the average of the total data access times becomes long, and as a result, the effect of reducing the data access time cannot be significantly improved.